This invention relates to coated abrasives and more particularly to coated abrasives in which the abrasive surface comprises a plurality of generally regular composite structures each of which comprises abrasive grain dispersed within a cured binder. The shape, spacing, size and composition of the composites can be manipulated to achieve a wide range of abrasive properties and for this reason the products are frequently referred to as xe2x80x9cengineered abrasivesxe2x80x9d and this convention is adopted herein.
However for certain applications, the performance of such engineered abrasives can be quite disappointing, falling short of the significant advantages realized in others. One such application is that obtained in wet grinding metals using relatively fine grit sizes. It has now been found possible to devise formulations that yield significantly improved results even though, from first principles, inferior results might be anticipated.
The present invention provides a coated engineered abrasive having an engineered abrasive surface comprising a plurality of shaped abrasive structures adhered to a backing material wherein the structures comprise a cured formulation comprising an acrylate-based binder resin with abrasive particles uniformly dispersed in the binder wherein the proportion of resin in the formulation is from 58 to 75%, and preferably from 60 to 72%, and most preferably from 65 to 68% by volume.
The binder resin component of the formulation is understood to include the polymerizable components as well as any curing agents used to accelerate or promote cure and adhesion control additives. The remainder comprises the abrasive particles as well as any fillers used to adjust the rheology of the cured formulations, lubricants and any solid additives such as grinding aids and other property-modifying solid materials. Other components can also be present including a polymer in an amount up to 60% of the volume of the curable binder resin which serves to modify the physical properties of the formulation.
In conventional engineered abrasives developed for dry grinding applications, optimum performance is achieved when the abrasive structures comprise approximately 55% of cured resin component, 28% filler and about 17% abrasive grain, all proportions being by volume. This is considered the best for holding the maximum volume of abrasive particles for performing the abrasive function. It is considered intuitively that decreasing the volume proportion of abrasive in favor of the binder resin would decrease the effectiveness of the abrading action. It is however surprisingly found that this is not the case. The effectiveness in certain applications, including wet grinding (which is understood to mean grinding with the application of a liquid lubricant such as a water, or oil, based liquid lubricant) actually becomes more effective both in terms of the amount of material cut in a designated period or in terms of the finish remaining on the abraded surface after the abrading action. It is believed that the improvement may also be seen in applications such as in moderate to high pressure, dry applications such as weld blending and metallic surface pit removal.
The binder resin component for which this surprising effect is manifested is based on polymerizable acrylate monomers and this is understood to mean polymers based on polymerizable mono-acrylates, di-acrylates, tri-acrylates and other polyacrylates as well as mixtures thereof, optionally further comprising oligomers such as polyesters and urethanes copolymerizable with such acrylates and copolymerizable monomers that can be used to adjust the degree of cross-linking or rheology of the finished polymer.
It is also found that if a further polymeric component is added to the formulation the beneficial effect is maintained and may even be enhanced in that the most advantageous results appear to be obtained towards the upper end of the above specified range. This appears to be true whether the added polymer is a thermoplastic such as PVC or a thermosettable resin such as a phenolic resin. The amount of such polymer that may be added can be up to 100% of the volume of the binder resin but is preferably from 10 to 60% and preferably from 20 to 40% by volume of the binder resin volume.
The abrasive grits used can be any of those that have been described in the context of engineered abrasives including fused or ceramic alumina, alumina-zirconias, silicon carbide, cubic boron nitride, diamond, ceria, silicon nitride and mixtures thereof. In some cases very mild abrasives such as gamma alumina, boehmite, silica or ceria can be used alone or in admixture with one or more other abrasives. The abrasive particle sizes commonly used with engineered abrasives often are finer than those used in conventional abrasives such that, average particle sizes ranging from 1 to 200 micrometers and preferably from 5 to 100 micrometers can be used. With the finer grits the finish obtained is often as critical as the aggressiveness of the material removal. Here too the formulations of the present invention prove to be surprisingly effective in that smoother finishes are secured than with more conventional formulations.
The volume of abrasive grits in the formulation can be from 5 to 30% and preferably from 10 to 25%, based on the volume of the formulation. Where the formulation includes a mineral filler, the amount of such filler can be up to 40% and preferably from 5 to 30%, of the volume of the formulation.
The formation of the engineered abrasive surface can be by any of those techniques known in the art in which a slurry composite of abrasive and a binder precursor is cured while in contact with a backing and a production tool so as to be adhered on one surface to the backing and to have imposed on the other surface the precise shape of the inside surface of the production tool. Such a process is described for example in U.S. Pat. No. 5,152,917 issued on Oct. 6, 1992, to Pieper, et al.; U.S. Pat. No. 5,304,223 issued on Apr. 19, 1994, to Pieper, et al.; U.S. Pat. No. 5,378,251 issued on Jan. 3, 1995, to Culler, et al.; and U.S. Pat. No. 5,437,754 issued on Aug. 1, 1995, to Calhoun, all of which are incorporated herein by reference. Alternative formation methods, including rotogravure coating, are described in U.S. Pat. No. 5,840,088 issued on Nov. 24, 1998, to Yang, et al.; U.S. Pat. No. 5,014,468 issued on May 14, 1991, to Ravipati, et al.; and U.S. Pat. No. 4,773,920 issued on Sep. 27, 1988, to Chasman, et al., and embossing techniques as described in U.S. Pat. No. 5,833,724 issued on Nov. 10, 1998, to Wei, et al.; and U.S. Pat. No. 5,863,306 issued on Jan. 26, 1999, to Wei, et al., may be used and these too are incorporated by reference in this application.